Based only on vocal ability alone it would be Dio, but all the intangibles that go with the music biz Ozzy wins in a landslide. He was part of the creation of heavy metal with Sabbath, he introduced the world such amazing talents like Randy Rhoads, Zakk Wylde and Randy Castillo. He headlined the longest running and most successful yearly concert tour in history. Ozzy has sold way more albums and puts on a great performance getting the crowd into it no matter what. People have come out in masses for Ozzy for over 40 years. And I'm not meaning anything terrible about this but longevity and survival goes to Ozzy as well. By all accounts out of the two, Ozzy should have died several times. He is the ultimate survivor in the biz and out.